the Royal Navy put a net over the vessel to catch bodies floating out of the ship. This was a common practice with Royal Navy vessels, and one that would be repeated during World War II with, for example, the sinking of the battleship HMS Royal Oak at Scapa Flow in 1939.
Paul and I turned the dive here at the sheared-off fo’c’sle deck. There was no point venturing out into free water here – we knew the bow section was missing and lay almost a mile away.
We moved aft down the port side of the wreck, past more empty lifeboat davits, the three funnel openings and the skeletal one-storey deckhouse from which they rose. As we moved aft we began to see the torch beams of the other divers moving here and there like light sabres, the divers themselves invisible in the darkness.
As we got to the very stern we found more scattered 4-inch shells beside an empty 4-inch gun mount. Was this the mount for the 4-inch gun which had been fired after the torpedo hit and had gone over the side taking its crew with it?
Moving round the stern, I shone my torch downwards and could see the three-bladed starboard prop in free water where the tide had created a scour pit round the stern of the ship. I traced the free section of shaft forward from its support bearing until it disappeared into its hull tube and forward to the engine room.
After 25 minutes exploring her remains, Paul and I called the dive and began to scooter back to the downline to ascend. The downline was easily found off the starboard side – a number of strobes were flashing away on it 5 metres off the wreck in the gloom. We retrieved our reel and wound in our line as we moved towards the downline and began to ascend.
As we rose above 50 metres, our surroundings began to get brighter again. We were rising out of the cocoon of darkness that shields Pathfinder. Then, at about 40 metres we seemed to pop out of the cloud of silty gloom into bright water. We suddenly had 20–30-metre visibility again.
We reached the transfer line and moved slowly across it towards the trapeze that we could see hanging in the water high above us. As we rose we started slowly going through our decompression stops, all the time moving towards the trapeze. As we got shallower, every now and then one of the other divers would appear from the gloom far below us, carefully carrying out their own deco stops.
Finally, the last diver up disconnected the transfer line and we all began to drift under the trapeze, moving slowly upwards as we carried out our deco stops at 12 and 9 metres before the long hang at the last stop, 6 metres. As it was getting a bit busy on the trapeze, Paul and I came off the trapeze and whiled away the deco time circling the other divers on our scooters.
Here at the end of the first dive, it is perhaps the right time to explain, in case you’re new to this, a little about breathing gases and decompression, to start breaking you in gently!
The open-circuit (OC) divers in our group arriving at the deco station were breathing from standard diving regulators, where, as you breathe out, your exhaled breath is vented as bubbles from your regulator that rise up to the surface.
As the dive was deeper than the safe recommended limit for diving standard compressed air, they were using a helium-rich breathing gas for the deep part of the dive, known as bottom mix. As they ascended at the end of the dive and began their decompression stops at about 20 metres or shallower, they were able to switch over to a cylinder of enriched air nitrox (EAN) slung under their arm on their webbing and designed purely for use during decompression; it is called deco mix. Perhaps it is best if I also explain a little about diving gases and accelerated decompression at this early stage.
Basically, the more oxygen in your deco mix, the more you can shorten – or accelerate – your decompression stops. But there are certain depth limitations for different levels of oxygen in your deco mix, as these increased oxygen levels when you are diving can be dangerous at different depths.
The air you are breathing just now, reading this book on the surface, is comprised of 79 per cent nitrogen and about 21 per cent oxygen. Although largely inert on the surface, at high pressure levels nitrogen has a narcotic effect – the nasty diving problem called nitrogen narcosis. So both of the elements that make up ordinary air, nitrogen and oxygen, can become problematic when you are diving deep.
Nitrogen narcosis is a creeping (and at first largely unnoticeable) debilitating effect, which starts for me (when I’m diving on air) at a depth of about 30–35 metres. You need to know a little about the mechanics of diving to understand how it becomes a problem.
As a diver descends, the increasing weight of water surrounding them tries to compress internal air spaces such as their lungs, which are, simplistically, just bags of air. Imagine taking an air-filled crisp bag down underwater – it would very quickly be compressed to a fraction of its size by the surrounding water pressure. To avoid this eventually fatal effect happening to a diver’s lungs, an aqualung (or breathing regulator) delivers increasing amounts of compressed air with each breath as they descend. The aqualung delicately and rather cleverly keeps the air pressure in the diver’s lungs exactly equal to the increasing water pressure around the diver. The lungs stay the same size as topside, and no catastrophic collapse happens.
Once a diver has descended to a depth of 10 metres, the weight of the surrounding water in which they are immersed is conveniently exactly equal to the weight of the whole atmosphere that presses down upon us whilst we are standing on land at sea level. On the surface, the weight of the atmosphere (atmospheric pressure) is called one atmosphere or one bar. So, adding the 1 atmosphere weight of the atmosphere itself to the 1 atmosphere weight of water at 10 metres produces a pressure (water pressure) of 2 bar (or 2 atmospheres): at 10 metres, the water pressure is exactly double the air pressure we experience on the surface. The doubled weight of water and atmosphere above the diver will compress the volume of any air spaces such as lungs to half its normal size if an aqualung is not used.
To combat this ‘squeeze’ as the old hard-hat divers called it, at a depth of 10 metres a diver’s aqualung feeds them air at twice atmospheric pressure, that is at 2 bar. The delicate equilibrium between the air pressure in the lungs and the surrounding water pressure is maintained.
At a depth of 40 metres the water pressure is five times atmospheric pressure – that is, 5 bar – and comprises the 1 bar (atmosphere) on the surface plus 1 bar (atmosphere) for each of the four 10 metres. Any air spaces such as lungs would be compressed to a fifth of the volume they would be on the surface – not good. So, the aqualung again cleverly feeds a diver air that is compressed to five times atmospheric pressure – 5 bar. Again, the air pressure in the diver’s lungs is kept exactly the same as the surrounding water pressure – and the diver’s lungs remain exactly the same size as on the surface.
Boyle’s Law – the law of inverse proportions – governs this effect. When scientists were trying to work out what happened to air underwater, some brave, hardy men would sit in an upturned barrel cut in half, which was lowered into the water. As the barrel was taken below to predetermined depths, the air inside was compressed and the water level rose. Marks would be made on the side of the barrel at different depths. The depths and compression marks were correlated, and the law became clear.
If each breath the diver takes holds five times as much air as normal (compressed into the same volume), the diver is absorbing five times as much of the individual constituents. Therefore, in every breath the diver breathes in five times as much nitrogen, and five times as much oxygen.
Nitrogen is largely inert on the surface; the 79 per cent of nitrogen you are breathing right now as you read this is passing in and out of your body harmlessly. But the deeper you go, the higher are the volumes of compressed air breathed in each breath – and the more the increasing amounts of nitrogen in your body start to cause the debilitating effect known as nitrogen narcosis. Cousteau with typical flair called this effect the ‘Raptures of
